Penetrating power of different types of radiation.
Gamma rays are the most energetic and shortest-wavelength type of electromagnetic radiation. They lie beyond X-rays on the electromagnetic spectrum and have wavelengths less than about 0.01 nanometer. Gamma radiation frequently accompanies alpha and beta emissions and always accompanies nuclear fission. Gamma rays are very penetrating and are best stopped or shielded by dense materials, such as lead or depleted uranium.
In many cases, the ejection of an alpha or beta particle completes the radioactive decay process. However, in some other cases the daughter nucleus when formed has too much energy to be stable and gets rid of this by the emission of a gamma ray, usually within thousandths of a second. Gamma rays cause ionization of atoms but in a different way to alpha and beta particles. Gamma rays have between 1,000 and 1,000,000 times the energy of visible light, which is sufficient for them to be able to remove electrons from atoms.
Gamma rays are much more penetrating than charged particles. For example, whereas a beta particle emitted by I-131 (a radioactive isotope of iodine) has a range in tissue of a few millimeters, the accompanying gamma ray can pass right through the body.
Gamma rays are given off by certain kinds of high-energy objects in space. The most powerful of these are gamma-ray bursters.
Discovery of gamma rays
Paul Villard, a French physicist working in Paris at the same time as Marie and Pierre Curie, is credited with discovering gamma rays. In 1895, Wilhelm Röntgen discovered X-rays and shortly thereafter Henri Becquerel discovered radioactivity of uranium salts.
Paul Villard's main interest was in chemistry, which guided him into his studies of cathode rays, X-rays, and "radium rays." His experiments in radioactivity led to the unexpected discovery of gamma rays in 1900. Villard recognized them as being different from X-rays because the gamma rays had a much greater penetrating depth. He had found they were emitted from radioactive substances and were not affected by electric or magnetic fields.
When the nuclear pile was invented and, after World War II, used for peaceful purposes, a new source of gamma rays became available. The isotope of cobalt known as cobalt-60 was for the first time produced in commercial quantities.